Absorbable surgical sutures of polyglycolic acid have recently been made available to surgeons.
As set forth in U.S. Pat. No. 3,297,033, polyhydroxyacetic ester is sometimes referred to as polyglycolide, or poly(glycolic acid) and can be considered as essentially a product of polymerization of glycolic acid, that is, hydroxyacetic acid, which in simplified form is shown by the equation: ##STR1##
For use as a suture, preferably n is such that the molecular weight is in the range of 10,000 or more. Above 100,000 the polymer is difficult to extrude.
In these molecular weight ranges the polymer has a melt viscosity at 245.degree. C. of between about 400 and about 27,000 poises. Because the fiber is from a synthetic and controllable source, with a controlled molecular weight and controlled small percentage of comonomer, the absorbability, stiffness and other characteristics can be modified. In general, the higher the molecular weight, the slower the rate of absorption under a given set of conditions.
Among several methods by which polyhydroxyacetic ester can be prepared, one preferred route involves the polymerization of glycolide, ##STR2## the cyclic dimeric condensation product formed by dehydrating hydroxyacetic acid. During polymerization of glycolide, the ring is broken and straight-chain polymerization occurs. Probably at least a small portion of the polymerization involves the formation of anhydride or ether linkages from a condensation of glycolic acid in a head-to-head, or tail-to-tail direction. The current state of the art is not sufficiently advanced to show with certainty the ratio of anhydride or ether linkages to ester group but indicates there are no more than a few percent of the total. A small quantity of methoxyacetic acid, ##STR3## or methyl hydroxyacetic ester, ##STR4## or their homologs, such as higher alkoxyacetic acids, or alkyl hydroxyacetic esters may be present during the polymerization as an end group stabilizer controlling the molecular weight and viscosity. Small quantities of other materials may be present in the chain, as for example d,l-lactic acid, its optically active forms, homologs, and analogs.
Said U.S. Pat. No. 3,297,033 incorporates a reference to U.S. Pat. No. 2,668,162 -- Lowe which quantifies a small amount of lactides as up to 15%, disclosing for example the preparation of a copolymer of 90/10 glycolide/lactide offers two advantages over the homopolymer of glycolide. One advantage is that the melting point of the copolymer is lower than the homopolymer, being in the neighborhood of 200.degree. C.; and the entire reaction can be conducted at approximately the melting point of the copolymer. Operation at the lower temperatures decreases the rate of degradation of the polymer which gives a polymer of lighter color. Another advantage is that the copolymer can be successfully quenched when being extruded into film because the copolymer is less crystalline. On the other hand, the homopolymer shows a greater tendency to crystallize on extrusion and thereby tends to form opaque areas in the film.
Example 4 of said U.S. Pat. No.2,668,162 shows reaction conditions.
Surgical elements of polyglycolic acid, including sutures, and other elements mentioned below can be better seen in most surgical fields if the element is colored so as to contrast with blood and tissue or bandages or other background materials. It is desirable that such elements be colored during manufacture with a dye or pigment which has no surgical disadvantages of its own, and which either has no effects on the polymer or improves its characteristics.
Colorants such as iron oxide or carbon black have been used to color non-absorbable sutures of polymers such as nylon or linear polyethylene or isotactic polypropylene. As such polymers are not absorbed, the colorant is removed when the suture is removed so completely non-absorbable pigments are satisfactory.
With an absorbable suture, as the suture is absorbed, the colorant is released into the living mammalian tissue, and its toxicity and metabolic fate become a matter of concern. It is necessary that any color component be tissue compatible. Usually, it is desirable that the colorant disappear to avoid tattoo traces in the tissue after the wound heals and the suture is absorbed.